Polychlorinated Dibenzodioxins and Polychlorinated Dibenzofurans - PSL1

Summary

Polychlorinated dibenzodioxins (dioxins) and polychlorinated dibenzofurans (furans) are highly persistent compounds with a strong affinity for sediments and a high potential for accumulating in biological tissues. They have been found in all compartments of the ecosystem, including: air, water, soil, sediments, animals and foods. All animals and humans in Canada are exposed to these substances.

Dioxins and furans enter the environment as complex mixtures from four major sources: commercial chemicals (eg. pentachlorophenol); incineration; pulp and paper mills that use chlorine bleaching; and both accidental fires and spills involving polychlorinated biphenyls (PCBs, which contain principally furan contaminants).

Both the number of chlorine atoms and their positions on the molecule determine the properties of dioxins and furans. It is primarily those dioxins and furans with chlorines in the 2, 3, 7 and 8 positions that are retained by animals and humans, and which concentrate selectively in body fat and fatty organs such as the liver. Although dioxins and furans can be metabolized and excreted, this is a relatively slow process in humans, with half-lives of several years reported for some dioxins and furans.

The compound 2,3,7,8-tetrachlorodibenzodioxin (and to a lesser extent, the other dioxins and furans substituted in the 2, 3, 7 and 8 positions) is extremely toxic to mammals, with a wide variation in sensitivity among species. In animals, death results from exposure to amounts ranging from less than one microgram to a few milligrams per kilogram of body weight. The bird and fish species tested are more sensitive than most mammals to short-term exposures to dioxins. Longer-term exposure of test mammals to lesser amounts can affect reproduction, cause birth defects, damage the liver and suppress the immune system. Exposure to 2,3,7,8- tetrachlorodibenzodioxin at certain doses causes cancer in rodents.

The ecosystem has clearly been affected in several well documented cases. Deaths of animals were noted after accidental exposures at Seveso, Italy and in Missouri, U.S.A. Adverse effects on reproduction of fish-eating birds have been linked to high ambient exposures in the Great Lakes and on Canada's West Coast. Malformed embryos and reproductive failure have been observed in fish-eating birds in the Great Lakes.

Studies of human populations indicate that exposure to several milligrams of mixtures of dioxins and furans can lead to a variety of effects on skin, eyes, and sensory and behavioural processes. Exposure of women to several milligrams of furans in contaminated rice oil in Japan and Taiwan may have been responsible for reproductive anomalies and infant mortality. To date, there has been no adequate demonstration that human populations exposed to dioxins and furans have suffered excess cancer.

Lifetime daily doses of 1 nanogram of 2,3,7,8-tetrachlorodibenzodioxin per kilogram of body weight have no effect on the incidence of cancer in rodents and did not affect fertility, litter size, fetal resorption and organ function in rats exposed through three generations. This dose is considered to be the no-observed-adverse-effect-level for 2,3,7,8-tetrachlorodibenzodioxin.

A recent Federal-Ontario multimedia study has estimated the average total exposure of Canadian adults and children to dioxins and furans from all pathways. These estimates are based on the average intakes and representative concentrations of these substances in the media to which Canadians are exposed. The average daily Canadian intake of dioxins and furans over a lifetime is estimated to be between 2.0 and 4.2 picograms [see page ix] of 2,3,7,8-tetrachlorodibenzodioxin toxic equivalents per kilogram of body weight per day. Virtually all of this intake comes from food.

Based on the no-observed-adverse-effect-level of 1 nanogram per kilogram of body weight per day and a 100-fold uncertainty factor, it is concluded that human intakes should be below 10 picograms toxic equivalents per kilogram of body weight per day averaged over a lifetime. This value may also be applicable to populations of wild mammals. While the estimated average Canadian exposure to dioxins and furans is below this 10 picogram intake level, persons eating highly contaminated fish in quantities well in excess of the general population norm may approach or exceed the guideline for intake. Wildlife that consumes highly contaminated fish may also be taking in a dose of dioxins and furans that exceeds their tolerable daily intake. Elevated concentrations of these substances in some shellfish near some pulp and paper mills that use chlorine bleaching have necessitated closure of the affected fisheries in order to protect human health. Estimates of dioxin and furan intakes by infants through breast milk are also relatively high. Although medical opinion is that the known benefits of breast feeding outweigh any potential risks, further contamination of human breast milk may lead to significant and unacceptable exposures to dioxins and furans.

Some dioxins and furans are very persistent, and continued release of these chemicals into the environment could unnecessarily prolong exposures, with a resultant increase in the risk to the environment and to human health. Those chemicals which are sources of dioxins and furans are already subject to stringent controls. However, dioxins and furans continue to be released from other sources such as incinerators which employ old technology and from pulp mills that use chlorine bleaching. Large quantities of PCBs (which are a potential source of furans) are currently in storage awaiting the establishment of suitable destruction facilities, which are starting to become available in Canada.

The most significant dioxin sources are the wood preservative pentachloro-phenol, municipal incinerators, and pulp and paper mills using chlorine for the bleaching process. Polychlorinated biphenyls (PCBs) are the most significant potential source of furans.

Based on these considerations the Ministers of the Environment and of National Health and Welfare have concluded that polychlorinated dibenzodioxins and polychlorinated dibenzofurans may enter the environment in quantities which have immediate and long-term harmful effects on the environment, and which constitute a danger in Canada to human health. These substances are therefore considered ^"toxic" as defined under Sections 11(a) and 11(c) of the Canadian Environmental Protection Act.

1 picogram = 1 x 10^-12 grams

1 nanogram = 1 x 10^-9 grams

1 microgram = 1 x 10^-6 grams

1 milligram = 1 x 10^-3 grams